The four stroke internal combustion engine is basically a pump. It must pump an air and fuel mixture in and pump the burned exhaust out. In order to increase engine efficiency and torque steps can be taken to enhance the pumping characteristics of the engine. One of the areas of engine tuning that helps in this regard is the exhaust system.
When the exhaust valve first opens, referred to as blow-down, the pressure remaining from the power stroke is released into the exhaust system. The resulting high-pressure wave travels down the system at the local speed of sound in the range of 600 meters per second. That speed depends upon engine and exhaust system design. Waves traveling through ducts will reflect back up the duct in the opposite direction whenever there is a change in cross sectional area of the duct. If the area is increasing then the wave will reflect as a negative of the original wave. A high-pressure wave will reflect a low pressure or rarefaction wave in such a case. If the area is reducing then the wave reflected is same sign as the original, which means that a pressure wave will reflect another pressure wave.
These waves moving back and forth present the opportunity to use them for improved engine performance. In particular there is a period in the engine's cycle when it is useful to have a rarefaction wave present at the exhaust valve. This period is during the exhaust stroke and the top dead center period between the exhaust stroke and intake stroke. A low pressure at the exhaust valve during this time assists in the exhaust process and can result in reduced residual exhaust in the combustion chamber to dilute the incoming air.